Events during 2018 could affect your withholding status or number of exemptions.
If you get divorced, you must give your employer a new Form W-4 within 10 days. For any other event that changes your withholding (getting married, having a new child, you can give your employer an updated W-4 at any time of the year. We recommend doing it as soon as possible after the event so the correct amount of tax can be withheld each pay period.
If you do forget to submit a new W-4 when the event occurs, be sure to submit it to your employer no later than December 15th. This will help ensure that the changes are reflected in the current year.